When two monosaccharides are put together to form one disaccharide how many water molecules are formed?

One water molecule is formed.

The process where 2 monosaccharides joined together is called condensation, and because after the bond, 2 hydrogen and 1 oxygen molecule is left out, therefore 1 water molecule is produced.

Note that when the new product disaccharides is broken down back into 2 monosaccharides, the water molecule is used back.